Inner layer displacement measuring method and apparatus



Oct. 8, 1968 w. E. CHOPE 3,405,267

INNER LAYER DISPLACEMENT MEASURING METHOD AND APPARATUS Filed June 17,1964 DETECTOR R SPONSE v RUBBER 2 u: can.

WEIGHT PER UNIT AREA,-

B5583? DETECTOR R B ER. RESPONSE R B 1 CORDS 2 DETECTOR CURRENT c WEIGHTPER UNIT AREA,X

I COMPUTER 2.; I f 54 7 INVENTOR w WILBERT E. CHOPE AGENT CORDS z UnitedStates Patent 3,405,267 INNER LAYER DISPLACEMENT MEASURING METHOD ANDAPPARATUS Wilbert E. Chope, Columbus, Ohio, assignor to IndustrialNucleonics Cor oration, a corporation of Ohio Continuation-impart ofapplication Ser. No. 24,105, Apr. 22, 1960. This application June 17,1964, Ser. No. 375,794

17 Claims. (Cl. 25083.3)

This is a continuation-in-part of my copending application (nowabandoned) Ser. No. 24,105 filed Apr. 22, 1960, and entitled RadiationThickness Gauging Technique for Measuring the Symmetry of Overlayers.

This invention relates generally to coating thickness gauges and moreparticularly to radiation gauging method and means for determining therelative position of an inner layer of one material in between outerlayers of a different material.

In the manufacture of many products in sheet form,

it is desirable to apply a coating of one substance on another toachieve improved properties in the combined materials which were notavailable in either material alone. One example is the manufacture ofvehicle tires in which the carcass of the tire body is built up ofrubberized plies which consist of rubber coated cord fabric. Thestrength of the tire carcass is contributed almost wholly by the cordmaterial whose strands are loaded in tension. However, in order toprotect the individual cords from impinging upon each other and thusundergoing abrasive wear, the fabric is encased in a layer of rubberforced between the individual cords of the fabric in the plane of thefabric and deposited as a protective layer both above and below theplane of the fabric. These latter layers protect the cords from theabrasive action of adjacent plies of which the tire is constructed andthe rubber between the cords within the individual ply protect thosecords from abrasive action between cords of the sam ply.

Because of the foregoing considerations of tire manufacture, it is ofgreat importance that the rubber applied to the cord fabric of the tireply be coated uniformly. If this condition is not achieved, a greateramount of rubber would be applied to one surface than the other. Thesurface with the lesser amount of rubber wou d expose the cords to agreater risk of abrasive wear; and the surface with the extra rubbercould lead to excessive build up of heat in the tire. The build up ofheat may be attributed to the relatively poor heat conductivity of theprotective rubber layer to the heat generated by flexure of the cord innormal service.

Because of the importance of insuring that the tire cord lies in thecenter of the protective rubber, it has been past industrial practicefor samples of the coated material to be cut periodically from theoutput of a calendering process. The machine operator then judges thebalance of the coatings applied to each surface of the cord fabric andmakes needed machine adjustments. In some instances the judgment is donevisually and when mechanical meastiring devices are employed theprocedure is destructive in that the sample is cut from the production.It is subject to errors in the operators judgment of the balance ofapplication of the coatings and it is a very small sample of productionon which to base a decision for machine adjustment.

Other industrial coating applications require either a balanced coatingor a coating weight ratio of a desired value to be applied to opposedsurfaces of a traveling sheet or other member.

Briefly I determine when the desired balanced coating condition prevailsby measuring the weight per unit area of the top coating and the bottomcoating by means of "ice radiation back-scattered from opposite sides ofthe sheet and computing any differences in the respective weights. Byeither subtracting or forming the ratio of the respective coatingweights, I provide a signal indicative of the relative deviation fromthe balanced condition. My method is particularly suited to betaradiation but other types of probing radiation might be used in certainindustrial applications. It is well known to those skilled in the artthat of the total radiation directed against an absorber a certainportion will be backscattered. The amount backscattered to a detector onthe same side of the sheet will increase as the weight per unit area xincreases in accordance with the following equation:

In Equation 1, R is the detector response, [L is the reflectioncoeflicient for the material being irradiated, and SR is the saturationresponse or that maximum response that results when the weight per unitarea x is very large.

At this point it is well to consider the relationship between the weightper unit area x, the density p and the thickness t of absorber. Bydefinition,

If the density of the absorber can be assumed to be constant, as it isin most applications, the aforesaid radiation backscatter gauge willmeasure the thickness of the absorber up to an infinite thickness valueT associated with the above mentioned very large value of x. Since theexponential term in Equation 1 approaches zero for large values of x,absorber thicknesses larger than the infinite thickness T will notchange the detector response R.

The concept of infinite thickness is a very important one and one onwhich the operability of most prior art radiation coating weight gaugescritically depends. To measure the thickness of a coating on top of abase material, such as zinc on steel, it is usually necessary in priorart gauging systems that the base material be infinitely thick to theincident radiation. Otherwise, the detector will respond to changes inthickness of the base as well as the coating.

My invention is not restricted by this limitation which cannot beprovided in most processes. By measuring opposite sides of the sheet Iexamine substantially the same region of the base material. The base canbe of any thickness up to infinite and larger but it must besubstantially invariant. As is the case in all backscatter coating gaugeapplications, the composition of the base and the coatings must beconstant and different; and, if the thickness, rather than the weightper unit area balance is to be measured, the density of both materialsmust not change substantially. I generate a first signal proportional tothe radiation backscattered from the top coating and a second signalproportional to the radiation backscattered from the bottom coating andI form the ratio of the two signals. If the signals are identical, thecomputed ratio value is unity. Ratios different from unity indicatethere is more coating on one side or the other or, in other words, thatthe base is positioned closer to one surface than the other surface ifthe density of each coating remains constant.

Accordingly, it is a primary object of the present invention to providea method and means for determining the relative balance of coatingapplied to opposite sides of a sheet. i

It is another object of the present invention to provide a measuringsystem for determining the position of cord fabric between two surfacelayers of different material.

It is yet another object of the present invention to provide a cordbalance measuring system that is continuous, nondestructive and moreaccurate than prior art systems.

It is also an object of the present invention to provide a cord balancemeasuring system that is simple to construct and economical to maintainin operating order.

It is still another object of the present invention to provide a cordbalance measuring system that is easily adaptable to existing processlines.

These and other Objects and advantages of the present invention willbecome more apparent upon reference to the following description whentaken in conjunction withthe drawings, in which:

FIG. 1 is a perspective view of a tire fabric manufacturing processmeasured for cord balance by backscatter gauges in accordance with thepresent invention;

FIG. 2 is a graph illustrating the response of a conventionalbackscatter detector to a single thickness of absorber;

FIG. 3 is a graph illustrating the backscatter gauge response to acoated base material for different values of base weight per unit area;

FIG. 4 is another graph similar to FIG. 3 but plotted for differentrelative values of atomic number of the base and coating materials;

FIG. 5 is a block diagram of the system shown in FIG. 1 illustrating ingreater detail the cord balance measuring techniques of the presentinvention; and,

FIG. 6 is a graph illustrating the different respective gauge responsesof the system of FIG. 5 to an unbalanced cord fabric.

Referring to the drawing and particularly to FIG. 1, there is shown atypical two calender train for making calendered fabric for use in themanufacture of automobile tires. A strip of fabric 10 formed of cords orother suitable material is fed in the direction of the arrows around aguide roll 11 to and between a center roll 12 and a lower roll 13 of afirst roll stand. Gum rubber or the like is distributed on an upper roll14 of the stand and transferred to one side of the fabric strip 10 bythe center roll 12 to form a rubber layer 15. The semi-coated sheet istrained around the lower roll 13 and fed to and between a center roll 16and a lower roll 17 of a second roll stand where another rubber layer 18is applied in a similar manner. The resulting coated fabric 20 emergesin the direction of the large arrow for further processing.

In accordance with the present invention the weight per unit area ofrubber coating on either side of the cord fabric 10 is measured by apair of radiation backscatter gauges 21 and 22 which are mounted invertical alignment adjacent to opposite surfaces of the coated fabric20. To determine cord unbalance, a difference computer 23 is coupled tothe gauges 21 and 22 by means of lines 24 and 25. The gauges may bestationary or of the traversing type adapted to scan the coated fabric20.

Each gauge includes a source of radiation directed toward the sheet 20and a detector positioned to receive radiation reflected back. Theamount of radiation reflected is proportional to the rubber weight perunit area in a manner described in detail hereinafter. A signalindicative of the weight per unit area of rubber on top of the cords istransmitted over line 24 and another signal proportional to the weightper unit area of rubber on the underside of the cord is transmitted overline 25. Difference computer 23 provides a signal on line 26 to anindicator 27 whenever the two signals are not identical in magnitude.

Referring to FIG. 2, either backscatter gauge is responsive not only tothe weight per unit area x of absorber in the path of radiation but alsoto its effective atomic number. The curves of FIG. 2 are each drawn fora different but constant effective atomic numbers, Z of the rubber and Zof the cords. The value Z does not necessarily identify a discreteelemental substance from the periodic table nor does Z These valuesalong the abscissa of FIG. 2 are the net effective Z of thehydrocarbonaceous material of which the rubber is composed and which isnormally loaded with appropriate amounts of sulphur and other additivesand the net effective Z of the cellulosiccarbohydrate material of whichthe tire cord may be com- 4 posed. Ideally, the difference in respectiveatomic numbers of the two materials should be as large as possible. Itmay be difficult for this qualification to be met in the tire plyprocess described herein unless either the cords or the rubber is loadedwith either a high or a low Z substance. When the weight per unit areaof absorber is zero there is some residual detector response SR due toradiation backscattering off of the air in the path of the incidentradiation; however, for all practical purposes this contribution to theoverall response can be ignored. For cords alone under thesource-detector, the response increases with the weight of the cords toa maximum saturation response SR If rubber instead of cords is placedunder the source-detector unit, a somewhat smaller saturation responseSR is noted if Z, Z

At a half-thickness, x of absorber the detected response will be exactlyone-half the value of the saturation response that occurs at infinitethickness, x It should be mentioned that although x is in units ofweight per unit area, it is proportional to thickness under theaforesaid conditions of constant density. Since the terms weight perunit area and mass are used interchangeably in the literature, I mayrely upon this definition in this disclosure. The saturation responsevaries with atomic number Z and atomic weight A according to :16 f th m:

where k is a constant of proportionally depending on the strength of theincident radiation, the geometry of the detector and other physicalparameters. Regardless of what Z material is being tested, a saturationresponse SR is reached at approximately the same value of x for a givenenergy range of incident radiation.

Having examined how the detector response varies when one material orthe other is irradiated, it is instructive to refer to FIG. 3 whichshows how the response changes when rubber is coated on top of the cordbase. The response curves of FIG. 2 are shown in dotted lines in FIG. 3to avoid confusion. In general, from Equation 1, the starting point ofeach curve is fixed by where x is the cord weight per unit area and SRis determined in accordance with Equation 3, and the response where .vcis the weight per unit area of rubber placed upon an x weight of cords.

If the cords are of a weight per unit area x =x then a response curve Rresults when increasing thicknesses of rubber are put on. In this case,the maximum response R =SR results when there is no rubber on the cords.It is in the heavier weight region xZx that prior art coating gaugesusually operate. However, the present invention works equally well belowthis backer weight; e.g., at x and x It may be noted that the responsecurves R and R are concave up while the response curve R is concavedown. The transition occurs at a cord weight x =x* where the detectorresponse is identically equal to the saturation response to rubber, SR,.At this critical weight per unit area of cords the response does notchange as more rubber is piled on because an infinite thickness ofrubber produces the same magnitude of response as zero thickness ofrubber. This anomalous situation can be shifted if a radiation source ofa somewhat different energy spectrum is selected. If senstivity isdefined as the ratio of a change of detector response to a change inrubber weight per unit area, it may be observed that the sensitivity'ofmeasurement improves the further the cord weight per unit area departsfrom the critical value x*.

It is recognized, however, that each backscatter gauge is examining athree-layer fabric and not only the cord layer and that rubber coatingnearest the surface but also the undercoating may contribute to theresponse of each detector. However, since the intensity of each incidentradiation beam is greatly reduced after passing through two thicknessesof absorber, the amount of radiation backscattered from the undercoatingwill be relatively small. Moreover, what little backscatter is generatedwould generally be substantially absorbed in the cord and surfacecoating in traveling back to the detector. Only when the cords are ofextremely light weight per unit area is it necessary to consider thesomewhat more complex threelayer response. When examining extremelylight cord weights the sensitivity of the measuring system deterioratesand this situation should be avoided. Accordingly, the remainingdescription for simplicity is presented as though each detector seesradiation backscattered only from the cords and that layer of rubberimmediately adjacent the detector. Referring now to FIG. 4, responsecurves are drawn for the condition that the atomic number Z of thecoating is greater in magnitude than the atomic number Z of the cords.If a relatively large cord weight per unit area x =x exists, then theresponse curve R results. It is apparent that the aforesaid anomalousopera-ting condition does not obtain when Z Z since the response curveswill all be concave up and there is no cord weight per unit area thatyields a detector response equivalent to the rubber saturation responseSR Referring now to FIGS. 5 and 6, the system of the present inventionis shown in more detail than FIG. 1. The top coating gauge 21 includes asource of beta radiation 30 surrounded by -a shield 32 and mounted inthe center of a detector 34. The detector comprises an ionizationchamber that generates an electrical current I in response to radiationreturned back from the coated fabric 2.0. An amplifier 36 drives anoutput current 1 through a potentiometer 38 to develop a potential 2 online 39.

The bottom coating gauge 22 includes a similar sourcedetector providinga current I into an amplifier 40. A potential e is developed across apotentiometer 42 by amplifier current I There is in effect provided onemeasuring channel inspecting the top of the cords and another thebottom. The detector currents which are plotted in FIG. 6 can be relatedto the response curves of FIG. 3 by the following relationships:

Since both detectors are operating along substantially the same responsecurve, when x =x the outputs of both channels will be identical. The twodetector-amplifier channels should also have essentially the sameoverall response characteristics. In most cases, an adjustment of eitherpotentiometer 38 or 42 should compensate for any difference in detectoror amplifier response. To check for balance, a sample of tire fabrichaving a precisely centered cord can be inserted between the detectors.With the sample in the gap This condition exists either when thedifference between the two signals is zero or their ratio is unity andcan be indicated by either a deviation meter 44 or a ratio indicator 46.Either presentation may be selected at will he means of a ganged switch48. In position #1, the switch connects e directly into a summingamplifier 50 but passes e through a phase-inverting unity gain amplifier52 before summing it with 2 In position #2, switch 48 connects the twopotentials into a ratio computer 54 of conventional design which drivesthe indicator 46.

From FIG. 6 it is apparent that if there is more rubber on the bot-tomthan the top of the tire ply, there Will be a somewhat smaller current Iin the lower channel than the current I in the upper channel. Similarly,the output potential e will be greater than 2 Meter 44 will indicate anegative value for deviation and the indicated ratio will be greaterthan unity.

It should be apparent that, if the detectors are vertically aligned asshown, radiation from one channel may directly pass through the fabricto the detector of the other channel. This is to be avoided because theextraneous direct radiation does not contribute any coating weightinformation and serves only to reduce the signal-to-noise ratio of thesystem. Of course, if the fabric is thick enough, substantially all theradiation incident on either side will be completely absorbed before itpasses through the fabric. Alternatively, gauge 22 may be laterallydisplaced to the dotted line position 22a so long as both detectors arelooking for all practical purposes at the same portion of the fabric.While coaxial source-detector units are illustrated, separate sourcesand detector assemblies may be utilized to direct and reflect radiationthrough any angle less than While a preferred embodiment of the presentinvention has been described, numerous changes, additions, and omissionsmay be made thereto without detracting from the original spirit andscope or relinquishing any of the advantages attendant thereto.

What is claimed is:

1. A method of determining the position in a sheet of one material ofconstant composition and density of an intermediate layer of differentmaterial but of constant composition and weight per unit area, saidmethod comprising the steps of:

directing radiation into opposite sides of said sheet,

and

correlating the difference in backscatter radiation from opposite sidesof said sheet with the position of said intermediate layer in saidsheet.

2. A method of determining the position in a sheet of one material ofconstant composition and density of an intermediate layer of differentmaterial but of constant composition and weight per unit area, saidmethod comprising the steps of,

directing beta radiation into opposite sides of said sheet,

correlating the difference in backscatter radiation from opposite sidesof said sheet with the position of said intermediate layer in saidsheet.

3. Apparatus for determining the position of a sheet of one material ofconstant composition and density of an intermediate layer of differentmaterial but of constant composition and weight per unit area, saidapparatus comprising:

means for directing a beam of radiation into opposite surfaces of saidsheet toward said layer,

means for detecting radiation returned from a first surface on one sideof said sheet to generate a first signal that is a function of thethickness of said sheet material located between said surface and saidlayer,

means for detecting radiation returned from a second surface on theopposite side of said sheet to generate a second signal that is afunction of the thickness of said sheet material located between saidopposite surface and said layer, and

means for combining said first and second signals in accordance with apredetermined relationship to obtain an output signal that is a functionof the deviation of said intermediate layer from the median plane ofsaid opposite sheet surfaces.

4. Apparatus for determining the position in a sheet of one material ofconstant composition and density of an intermediate layer of differentmaterial but of constant composition and weight per unit area, saidapparatus comprising:

means for directing a beam of beta radiation into opposite surfaces ofsaid sheet toward said layer, means for detecting beta radiationreturned from a first surface on one side of said sheet to generate afirst signal proportional to the thickness of said sheet materiallocated between said surface and said layer, means for detecting betaradiation returned from a second surface on the opposite side of saidsheet to generate a second signal proportion to the thickness of saidsheet material located between said opposite surface and said layer, and

means for measuring any differences in said signals to determine whetheror not said intermediate layer is equidistant from each of said sheetsurfaces.

5. Apparatus for determining the position of a layer of one material ofconstant composition and weight per unit area in a sheet of differentmaterial but of constant composition, said apparatus comprising:

means for directing a first beam of beta radiation into one surface ofsaid sheet toward said layer,

means for directing a second beam of beta radiation into an oppositesurface of said sheet toward said layer, means for detecting betaradiation backscattered from each of said surfaces to provide a firstsignal proportional to the thickness of said material between said layerand one of said surfaces and a second signal proportional to thethickness of material between said layer and said opposite surface ofsaid sheet, said layer material being less than an infinite thicknesswith respect to said beams of beta radiation,

means for computing the ratio of one of said signals to the other toderive an output signal proportional to the deviation of said layer fromthe median plane of said opposite sheet surfaces, and

means for registering an indication of said output signal, to identifythe deviation of said layer from the median plane of said opposite sheetsurfaces.

6. Apparatus for measuring the displacement of an intervening layer ofone material within a sheet of a different material, comprising:

means for directing into the first and second opposed surfaces of saidsheet respectively first and second beams of penetrative radiation;means including a first detector responsive to radiation from said firstbeam which is reflected backwardly from said first surface forgenerating a first signal which is variable in accordance with the massof said intervening layer, the composition of said layer, thecomposition of said sheet material, and the mass of said sheet betweensaid first surface and said layer;

means including a second detector responsive to radiation from saidsecond beam which is reflected backwardly from said second surface forgenerating a second signal which is variable in accordance with saidmass and said composition of said layer, said composition of said sheetmaterial and the mass of said sheet between said second surface and saidlayer;

means for balancing said signals when a sample sheet with itsintervening layer displaced as desired is disposed between said firstand second beam directing means and their respective detectors, toproduce mutual cancellation of the effects on both said signals of saidmass of said intervening layer, said composition of said layer and saidcomposition of said sheet; and

means for indicating any remaining unbalance in said signals when asheet to be tested as to the displacement of its intervening layer isdisposed between the first and second beam directing means and theirrespective detectors.

7. Apparatus for measuring relative to opposite surfaces of a sheet thedisplacement of an intervening layer of one material within said sheetwhich is of a different material, comprising:

first and second penetrative radiation sources for generating anddirecting first and second penetrative radiation beams respectively intothe first and second opposed surfaces of said sheet;

means including a first detector responsive to radiation from said firstbeam which is reflected backwardly from said first surface forgenerating a first signal which is variable in accordance with the massof said intervening layer, the composition of said layer, thecomposition of said sheet material, and the mass of said sheet betweensaid first surface and said layer;

means including a second detector responsive to radiation from saidsecond beam which is reflected backwardly from said second surface forgenerating a second signal which is variable in accordance with saidmass and said composition of said layer, said composition of said sheetmaterial, and the mass of said sheet between said second surface andsaid layer;

said first source and respective detector being displaced relative tosaid second source and its respective detector; means for initiallybalancing said signals, when a sample sheet with its intervening layerdisplaced as desired is disposed between said first and second sourcesand their respective detectors, to produce mutual cancellation of theeffects of both said signals of said mass of said intervening layer,said composition of said layer, and said composition of said sheet; and

means for indicating any remaining unbalance in said signals when asheet to be tested as to the displacement of its intervening layer isdisposed between the first and second sources and their respectivedetector.

8. Apparatus as set forth in claim 6, wherein said directing meansdisplaces said radiation beams to prevent the detection of radiationtransmitted directly through said sheet.

9. Apparatus for measuring the displacement of an intervening layer ofone material within a sheet of a different material, the weight per unitarea and composition of said layer and the composition of said sheetremaining substantially constant, said apparatus comprismg:

means for directing into the first and second opposed surfaces of saidsheet respectively first and second beams of beta radiation, meansincluding a first detector responsive to radiation from said first beamthat is reflected backwardly from said first surface for generating afirst signal which is variable in accordance with the weight per unitarea of said sheet between said first surface thereof and said layer,

means including a second detector responsive to radiation from saidsecond beam that is reflected backwardly from said second surface forgenerating a second signal that is variable in accordance with theweight per unit area of said sheet between said second surface thereofand said layer,

means for computing the ratio of said signals,

means for initially balancing said signals so that said ratio is unitywhen a sample sheet with its intervening layer displaced as desired isdisposed in the path of said first and second beam directing means, and

means for indicating any difference in said ratio when a sheet to betested is disposed in said path of said first and second beam directingmeans.

10. Apparatus as set forth in claim 9 in which said radiations directingmeans comprises:

first and second radioisotopes,

said radioisotopes being selected to provide a predetermined averageenergy for said first and second beams of radiation whereby neither ofsaid overlaying sheets presents more than an infinite thickness ofabsorber to said beams of radiation.

11. Apparatus as set forth in claim 9, wherein said directing meansdisplaces one of said beams of radiation from the other to preventdetection of radiation transmitted directly throughout the sheet. I

12. A method as set forth in claim 1 wherein said correlation stepincludes:

disposing relative to said radiation a sample sheet having anintermediate layer displaced as desired, and balancing the effects ofthe composition and the Weight per unit area of said intermediate layerand the composition of said sheet upon said correlated difference.

13. A method of determining the position of a layer of cords in a sheetof rubber of different composition, said method comprising the steps of:

directing beta radiation into opposite sides of said sheet,

detecting beta radiation backscattered from each of said opposite sheetsides, and

measuring the difference in said detected backscattered radiation toindicate the position of said cord layer relative to said opposite sheetsides.

14. Apparatus for measuring the position of a layer of cords in a sheetof rubber of different composition comprising:

means for directing radiation into opposite surfaces of said sheet tobackscatter radiation off of said cord layer,

means for detecting the difference in radiation backscatter from saidopposite sheet surfaces, and

means for indicating said detected difference in backscatter radiationto determine the position of said cord layer relative to said oppositesheet surfaces.

15. Apparatus for measuring the position of a layer of cords in a sheetof rubber of different composition comprising:

radiation source means for directing radiation into opposite surfaces ofsaid sheet to backscatter radiation off of said cord layer. radiationdetector means positioned adjacent to said opposite sheet surfaces togenerate a first and a second signal proportional to the amount ofradiation backscattered from each of said sheet surfaces, and meansresponsive to said signals to indicate the position of said cord layerrelative to said opposite sheet surfaces. 16. Apparatus as set forth inclaim 15 which further includes:

means for subtracting one of said signals from the other. 17. Apparatusas set forth in claim 15 which further includes:

means for computing the ratio of one of said signals to the other.

References Cited UNITED STATES PATENTS 2,855,518 10/1958 Foley et al.25083.3 X 2,897,371 7/1959 Hasler 250-83.3 X 3,148,279 9/1964 Skala250-83.3

ARCHIE R. BORCHELT, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,405,267 Dated ctober 8 1968 Inventor) Wilbert E Chope It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 5, equation 7, "I R =R should read I =R =R Column 5, line 60,"be" at the second occurrence should read Column 8, line 57, "beam"should read beams Signed and sealed this 15th day of December 1970.

(SEAL) Attest:

EDWARD M. FLETCHER,JR. WILLIAM E. SCHUYLER, JR Attesting OfficerCommissioner of Patents i FFIQM nnnnsn (ind-1Q!

3. APPARATUS FOR DETERMINING THE POSITION OF A SHEET OF ONE MATERIAL OFCONSTANT COMPOSITION AND DENSITY OF AN INTERMEDIATE LAYER OF DIFFERENTMATERIAL BUT OF CONSTANT COMPOSITION AND WEIGHT PER UNIT AREA, SAIDAPPARATUS COMPRISING: MEANS FOR DIRECTING A BEAM OF RADIATION INTOOPPOSITE SURFACES OF SAID SHEET TOWARD SAID LAYER, MEANS FOR DETECTINGRADIATION RETURNED FROM A FIRST SURFACE ON ONE SIDE OF SAID SHEET TOGENERATE A FIRST SIGNAL THAT IS A FUNCTION OF THE THICKNESS OF SAIDSHEET MATERIAL LOCATED BETWEEN SAID SURFACE AND SAID LAYER, MEANS FORDETECTING RADIATION RETURNED FROM A SECOND SURFACE ON THE OPPOSITE SIDEOF SAID SHEET TO GENERATE A SECOND SIGNAL THAT IS A FUNCTION OF THETHICKNESS OF SAID SHEET MATERIAL LOCATED BETWEEN SAID OPPOSITE SURFACEAND SAID LAYER, AND MEANS FOR COMBINING SAID FIRST AND SECOND SIGNALS INACCORDANCE WITH A PREDETERMINED RELATIONSHIP TO OBTAIN AN OUTPUT SIGNALTHAT IS A FUNCTION OF THE DEVIATION OF SAID INTERMEDIATE LAYER FROM THEMEDIAN PLANE OF SAID OPPOSITE SHEET SURFACES.